I will prospectively and randomly select persons for study from a population which will include persons with familial hypercholesterolemia and other forms of hypercholesterolemia. I will isolate their monocytes and incubate them in full and solvent extracted sera and determine the level of 3-hydroxy-3-methylglutaryl CoA (HMG-CoA) reductase activity. I will determine the sensitivity, specificity, and predictive value of this procedure in the detection of persons with familial hypercholesterolemia.